These days user's demand for more powerful computer systems grows constantly. Power supply requirements of CPU and peripherals also increase. The power supply used in the computer system also has to provide a greater power output. However, due to the concern of the possible harmful effect to human body, there is an energy limitation for the output power of power supply (maximum output power (Max VA) specification at present is mostly 240 VA. For example, with output of 12V, the maximum output current is 20 A). For instance, in SSI or UL standards, there is an Energy Hazard standard for the power supply of electric equipment. The maximum output power of the power supply is 240 VA to protect the safety of the general computer users.
Refer to FIG. 1 for the overload protection technique now adopted by the conventional power supply. The power supply receives AC power 10 and has commutation filter circuits 21 and 22 and a transformation circuit 31 to transform the AC power to a plurality of DC powers to drive at least one electronic device 81 at the rear end. The transformation circuit 31 outputs DC power of different voltage levels according to different coil number ratios on the secondary side. There is a control circuit 41 (consisting of a power switch 411 and a pulse-width modulation (PWM) controller 413) connecting to a driving signal source on the front side of the transformation circuit 31. The protection mechanism captures a power use value on the DC power cord input to the electronic device 81 and sends the power use value to a protection circuit 71. The protection circuit 71 provides a preset over-voltage protection value in a constant current or constant voltage mode to be compared with the power use value. If the power use value is greater than the over-voltage protection value, the protection circuit 71 outputs the comparison value to the power switch 411 or PWM controller 413 to stop or suspend power output on the DC power cord that drives the electronic device 81 at the rear end. Thus the object of the overload protection mechanism for the power supply is achieved.
However, nowadays the operation speed of computer system increases very fast, and the number of electronic devices connected to the computer system also increases constantly, abnormal overload condition could happen to one of the electronic devices. In such an occasion, the power supply immediately stops all power output. If the overloaded electronic device is a storage device or other peripheral (such as a backup hard disk or optical disk drive), operation of the computer system does not have to be stopped immediately. Moreover, when the power supply stops power output and causes shutdown of the computer system, users do not know which electronic device is overloaded. Users have to send the entire computer system for repairs. It is troublesome.